This invention relates generally to method and apparatus for producing color picture or color ink signals which are used in additive process of color reproduction by way of an ink-jet system, a color printing system or the like.
Color reproduction by way of an ink-jet system or a color printing system is effected such that three primary color inks, i.e. yellow, magenta and cyan inks, are respectively applied to a print sheet to be added to each other, while a black ink may also be added if desired. Namely, an original color of a picture or an image to be reproduced is first read by way of opto-electromechanical scanning to produce three primary color signals, and then the amount of three primary color inks is controlled in accordance with the ratio between these signals to paint or give color inks on a print sheet or the like. However, since currently available color inks are not ideal in connection with purity, color mixing control cannot be simply effected in accordance with the ratio between signals indicative of the original color. Describing in detail, yellow, magenta and cyan inks respectively include components of other colors, and therefore, these other color components result in impure or muddy color. Suppose a yellow component in the yellow ink, a magenta component in the magenta ink and cyan component in the cyan ink are respectively expressed in terms of 1, and other color components included in these primary color inks are expressed as follows.
magenta component in the yellow ink: k.sub.21 PA0 cyan component in the yellow ink: k.sub.31 PA0 cyan component in the magenta ink: k.sub.32 PA0 yellow component in the magenta ink: k.sub.12 PA0 yellow component in the cyan ink: K.sub.13 PA0 magenta component in the cyan ink: k.sub.23
In the above, the occurrence of impure or turbid colors cannot be avoided unless the value of the above components k.sub.12, k.sub.13, k.sub.23, k.sub.21, k.sub.31 and k.sub.32 is zero. Assuming that yellow, magenta and cyan components of a mixed color obtained as the result of color reproduction are expressed in terms of Yi, Mi and Ci, these components Yi, Mi and Ci can be given by: EQU Yi=Y.sub.0 +k.sub.12 M.sub.0 +k.sub.12 C.sub.0 EQU Mi=k.sub.21 Y.sub.0 +M.sub.0 +k.sub.23 C.sub.0 ( 1) EQU Ci=k.sub.31 Y.sub.0 +k.sub.32 M.sub.0 +C.sub.0
wherein Y.sub.0, M.sub.0 and C.sub.0 are the amounts of yellow, magenta and cyan inks to be applied and mixed; and it is assumed that an additive process is effected such that each color ink is mixed in proportion to the amount thereof.
The above formulas (1) are correct only when each color is simply mixed in proportion to the amount of the corresponding color ink, and represents the degree of color impurity, or impureness.
Since such color impureness caused by impure or turbid color components is undesirable, correction or compensation has to be effected. Up to this time, the above formulas (1) have been used to effect correction or compensation of impure colors. Namely, the values of Y.sub.0, M.sub.0 and C.sub.0 are respectively selected so as to satisfy the following formula (2). ##EQU1##
Although the above-mentioned conventional way of color correction in additive process is theoretically correct, it is not necessarily ideal in actual color printing or ink-jet color reproduction because each primary color ink does not mix with each other in proportion to its amount. In other words, the additivity rule is not valid. Especially in ink-jet color reproduction, it is difficult to mix each primary color ink with the others in proportion to its amount because of the following reasons. In order to ensure quick drying, a water-soluble ink is usually used, and such water-soulble ink is readily absorbed into a print sheet. As a result, the three primary color inks are difficult to mix with each other. In the case that the amount of three primary color inks applied on a print sheet is small, an additive process does not takes place satisfactorily. Furthermore, in order to ensure quick drying a sheet of paper having high absorbing power is often used. For these reasons, inks applied on the surface of a printing sheet are prevented from being mixed in an ideal manner. Generally speaking, a mixed color from two or three colors exhibits three primary color components whose values are lower than those derived from formula (1).